1. Field of the Invention
The present invention relates to tyres for motor vehicles. In particular, the present invention relates to tyres intended for fitting on the front wheel and/or on the rear wheel of sports/touring motor vehicles of large cylinder capacity (for example 1200 cm3 or more), and/or high power (for example 140-150 BHP or more).
2. Description of the Related Art
In general, a tyre for vehicle wheels comprises a carcass structure essentially composed of one or more carcass plies formed according to a substantially toroidal configuration and having the actual side edges axially opposed engaging with respective annular reinforcing structures incorporating metallic circular inserts, usually called “bead wires”. Each annular reinforcing structure is embedded in a so-called “bead” for anchoring the latter to a corresponding fitting rim.
A belt structure comprising one or more belt layers shaped as a closed ring, essentially composed of textile or metal cords suitably oriented relative to one another and relative to the cords belonging to the adjacent carcass plies, is applied on the carcass structure, in a radially external position.
A tyre tread, normally comprising a band of elastomeric material of adequate thickness, is further applied in a position radially external to the belt structure. The term “elastomeric material” typically comprises at least one elastomeric polymer and at least one reinforcing filler. Preferably, the elastomeric material further comprises additives, such as crosslinking agents and/or plasticizers. Owing to the presence of the crosslinking agents, on heating, said material can be crosslinked, so as to form the final article.
A pair of sidewalls each of which covers a lateral portion of the tyre between a so-called shoulder zone, positioned close to the corresponding side edge of the tyre tread, and the corresponding bead, can be applied on the opposite sides of the tyre.
Relative to the tyres for four-wheeled vehicles, the tyres for two-wheeled vehicles have to meet quite particular requirements, involving numerous structural differences. One of the most important differences arises from the fact that while travelling along a curve, a motorcycle must lean considerably relative to the position while travelling in a straight line, forming, with the perpendicular to the ground, an angle (called the camber angle) that can reach 45°, or even 65° in extreme driving conditions. Therefore, when the motorcycle goes into a curve, the area of contact of the tyre is displaced progressively from the central zone of the tread towards the axially outermost zone in the direction of the centre of the curve. For this reason the tyres for two-wheeled vehicles are characterized by their accentuated transverse curvature. This transverse curvature is normally defined by the particular value of the ratio of the distance between the radially outermost point of the tread and the line passing through the laterally opposite ends of the tread itself, measured on the equatorial plane of the tyre, to the distance measured along the chord of the tyre between said ends. In tyres for two-wheeled vehicles, the value of the curvature ratio is, in general, not less than 0.15, and is normally of the order of about 0.3 in the case of rear tyres, and even higher, up to about 0.45, in the case of front tyres, compared with a value only of the order of about 0.05 in motor vehicle tyres.
In tyres for motor vehicles, moreover, the belt structure can be provided by one or more continuous cords wound as turns axially close together and substantially parallel to the direction of circumferential development of the tyre itself (so-called “zero-degrees belt”).
Recently there has been a trend for motor vehicles for sports/touring use with ever increasing cylinder capacity and/or power to be marketed. In fact, for example, motor vehicles for road use with cylinder capacity of 1250 cm3, with power of 140-150 BHP, are already on the market.
The tyres fitted to the wheels of such motor vehicles must possess excellent qualities of road grip, for the high torque to be able to be transmitted to the ground, whether while travelling in a straight line, or especially while accelerating when coming out of a curve, as well as guaranteeing an effective braking action. Grip becomes a really critical point in conditions of driving on a wet road surface.
Together with excellent road grip, tyres must guarantee stable behaviour in particular while travelling in a straight line and the ability to last a high mileage. Stable tyre behaviour is in fact an indicator of its capacity for efficiently damping the perturbations transmitted from the irregularities of the road surface while travelling, so that these perturbations are not transmitted to the motor vehicle, compromising driving stability.
For better grip on the road surface it is possible to use soft compounds, which are able to adapt to the profile resulting from the asperities of the road surface and/or penetrate said asperities. These compounds are characterized by low elastic modulus and/or high hysteresis.
However, compounds that are too soft lead to reduced straight-line stability and lower mileage.
To overcome the aforementioned problems, tyres have been proposed with treads comprising various compounds.
U.S. Pat. No. 6,988,520 describes a motor vehicle tyre comprising a tread reinforced with a belt structure that has, in its normally inflated conditions, a curvature value C/L between about 0.5 and 0.7, a reinforced carcass ply that extends radially inwards relative to the belt stack and between the two bead regions and wound in each bead region around an annular bead wire from inside to outside in the axial direction to form a carcass that is backfolded and with the sidewalls of the tyre arranged between the edges of the tread and the bead regions. The belt structure comprises two belt plies comprising reinforcing cords inclined relative to the circumferential direction of the tyre, characterized in that the tread comprises two different compounds, the first compound that extends in a first layer between the edges of the tread and the second compound arranged in a second layer radially external relative to the first layer in the central portion of the tread.
EP 0,774,367 describes a motorcycle tyre that has, when normally inflated, a curvature value C/L of about 0.5 and 0.7. In the right axial section the tread comprises two tread rubber components of different compound, joined together by a scarf joint, said scarf joint having a width (JW) less than the tread width (TW) and an intertread ply comprising reinforcing cords interposed between the two tread rubber components of different compound.